In order to study the mechanism of tumor cell surface antigen shedding, galactosyltransferase levels were compared in 5 spontaneously metastasizing and 3 nonmetastasizing rat mammary tumors. The enzyme activity both with or without exogenous acceptors was higher in the metastasizing group. This difference did not seem to be due to the variation in levels of degrading enzymes such as pyrophosphatase or beta-galactosidase found in these tumors. Little difference in the biochemical properties of the enzyme was found between the two groups. Most of the enzyme activity (60-70%) was recivered in the microsomal frctosyltransferase was assayed in "purified" plasma membrane fractions, 70% of the activity was associated with the plasma membrane vesicles, in which the enzyme was enriched by factors of 10-40. The number of galactose acceptor sites on the plasma membranes increased in parallel to the metastasizing capacity, indicating the presence of larger numbers of incomplete glycopeptides on their cell surfaces. These findings seemed to indicate that the greater turnover of glycoprotein in the spontaneously metastasizing tumor cell surface was caused by the shedding of surface antigens into the systemic circulation of the host.